Henna Help!!!!

So...back to hair... ;) I have been reading and researching and reading some more about henna and the benefits. This is definitely something I want to try, but I must say that I am a wee bit hesitant, because let's face it folks I am soooo new to this hair thing. I think other than it's outward appearance, this is the most attention I have ever paid to my hair. The only reason I am hesitant is because plain and simple, I don't know what to do. I read all of the directions on hennaforhair.com and they seem simple enough, but what if I do it wrong???? I don't want green of fire engine red hair! My hair is a really dark brown or an "off black". I like having dark hair and would really want to keep it that way and I believe I can do that with henna and retain the benefits of strengthening my very fragile strands. I've read curlynikki's post about henna and I am building up the courage to give it a try.

If any of you wonderful henna using ladies out there have and tips for this newbie, please - POST!!! Let me know what's good, bad or in between about it. Thanks again....